Slipping-spark ignition plugs



SLIPPING-SPARK zcnmon PLUGS Filed April 1. 195a 2,961,563 SLlPPING-SPARKIGNITION PLUGS Lucien Pras, Billancourt, France, assig'nor to R'egi'eNationale des Usines Renault, Billancourt, France This invention relatesin general to spark ignition devices and more particularly to sparkplugs of the type wherein the spark is produced by discharging acapacity and wherein the electrical voltage or tension and the electrodegap have such values that the spark cannot take place in the atmospherewhere the ignition is required unless this spark remains in contact witha solid insulating member disposed between the electrodes. In theabsence of such solid insulating member the spark ill not fly unless aconsiderably higher tension is used and in this case either theadvantages resulting from caacity-discharge ignition must be waived orthe inter- -:::trode gap must be set at a very small value of the dcr of0.008" to 0.002"; now it is evident that this ondition is ratherdiflicult to meet, and furthermore as consequence of the electrode wearcaused by sparks ssurring at a high rate the interelectr-ode gap wouldrapidly increase to a value preventing the passage of parks.

it is already known, in their application to internal combustionengines, to constitute spark plugs the spark ing element of whichconsists of two electrodes spaced about 0.004" to 0.008" apart, theinter-electrode gap being occupied by a solid-supporting insulator flushwith the electrode surfaces. However, the useful life of capacitydischarge spark plugs of this character is reduced more or less by twomain and different causes, namely:

Electrode wear, whereby the relative spacing of the sparking edgesincreases until it is too great to enable the spark to fly between theseedges;

Premature wear of the insulator due to the thermal action exertedthereon by the sparks, whereby the sparks will progressively sink insidethe cartridge until they become unable to ignite the fuel mixture.

Now according to this invention it is possible to providecapacity-discharge spark plugs characterized by an extremely long usefullife.

This invention provides a method of manufacturing spark plugs of thetype defined hereinabove, wherein the insulator disposed between theelectrodes is rendered particularly apt to facilitate in a reliable andlasting manner the passage of sparks by being coated with an enamel ofspecial composition which has a high specific inductive capacity.

According to a typical form of embodiment of the invention, theinsulator, whether cured or uncured, consisting for example of sinteredalumina or clay-bound alumina, is coated on its face turned towards thechamber containing the fuel mixture to be ignited, with an enamelapplied wit-h a brush, by dipping or by any other adequate means, thisenamel consisting of a suspension in a liquid of the material in afinely divided state. After the coating step, the insulator is dried andbrought to the proper baking temperature in a suitable atmosphere.

Thus, after fitting the insulator in the assembly, a spark plug forcapacity-discharge ignition which has an insulator of adequate shape,enamelled on its operative portion, that is, on the portion situated inthe interelectrode gap on which the sparks are caused to fly and slip,is obtained. The insulator is be coated" with' the aforesaid enamel mayhave the following composition:

f Percent S19 S IQ 45 T 1 C50 0 t0 5 An enamel composition particularlysuitable for coating the active surface of an insulator of the kinddefined hereinabove has the following composition:

In this enamel the last four elements associated beforehand by bakingare in a vitreous condition. The oxides of titanium and barium, orbarium titanate are thoroughly mixed by crushing the vitreous phaseuntil a finely "divided powder is obtained. This powder in the form ofan aqueous suspension is then applied by means of a brush on the portionof the aluminous insulator which is to be enamelled. After drying, thecoated insulator is brought to the curing temperature of 800 to 1,350C., this curing operation being effected compulsorily in a reducingatmosphere, for example of hydrogen; the enamel is obtained to theproper degree when its color is dark blue and adheres perfectly on itsaluminous support.

The baking of the enamel may be carried out by means of a heating torchsuccessively in the oxidizing or reducing portion of the flame, or byusing any other adequate heating means of which at least the last phaseis a reducing one.

Satisfactory results are also obtained by carrying out the last heatingstep in a nitriding atmosphere.

An ignition spark plug of the slipping-spark type according to thisinvention is illustrated diagrammatically by way of example in thesingle figure of the attached drawing. In this figure, the referencenumeral 1 is the insulator, Z and 3 are the electrodes, and 4 the enamelcoating.

I claim:

1. A slipping-spark ignition spark plug having an insulator interposedbetween electrodes, the active portion of said insulator being coatedwith a baked enamel having a high specific inductive capacity, saidenamel being the nonvolatile residue of an aqueous suspension of theenamel components in a finely-divided state, said enamel consistingessentially of 5 to 30% TiO 20 to 46% BaO, 1 to 10% B 0 30 to 45% SiO0.5 to 3% A1 0 and 1 to 9% Na O.

2. A method of manufacturing spark plugs of the type wherein aslipping-spark is produced in contact with a solid insulator, whereinthe insulator interposed between the electrodes of the spark plug isrendered particularly suitable for the passage of the sparks, whichcomprises the steps of coating the active portion of said insulator withan enamel having a high specific inductive capacity, said enamel beingapplied as an aqueous suspension of its components in a finely dividedstate, drying the insulator and baking the enamel coating upon saidinsulator in a reducing or nitriding atmosphere at a temperature of 800to 1350 C., said enamel consisting essentially of 5 to 30% n0 20 to 46%BaO, 1 to 10% 3 B203, 1.0 S102, 0.5 to A1203 and 1 to N30;-

3. A method of manufacturing spark plugs of the type wherein aslipping-spark is produced in contact with a solid insulator, whereinthe insulator interposed between the electrodes of the spark plug isrendered particularly suitable for the passage of the sparks, whichcomprises the steps of coating the active portion of said insulator withan enamel having a high specific inductive capacity, said enamel beingapplied as an aqueous suspension of its components in a finely-dividedstate, drying the insulator and baking the enamel coating upon saidinsulator in a suitable atmosphere, said enamel consisting essen- 4tially of 5 to 30% TiO- 20 to 46% B210, 1 to 10% B 0 30 to 45% Si0 0.5to 3% A1 0 and l to 9% Na0 References Cited in the file of this patentUNITED STATES PATENTS a. haw

1. A SLIPPING-SPARK IGNITION SPARK PLUG HAVING AN INSULATOR INTERPOSEDBETWEEN ELECTRODES, THE ACTIVE PORTION OF SAID INSULATOR BEING COATEDWITH A BAKED ENAMEL HAVING A HIGH SPECIFIC INDUCTIVE CAPACITY, SAIDENAMEL BEING THE NON-VOLATILE RESIDUE OF AN AQUEOUS SUSPENSION OF THEENAMEL COMPONENTS IN A FINELY-DIVIDED STATE, SAID ENAMEL CONSISTINGESSENTIALLY OF 5 TO 30% TIO2, 20 TO 46% BAO, 1 TO 10% B2O3, 30 TO 45%SIO2, 0.5 TO 3% AL2O3 AND 1 TO 9% NA2O.